This invention relates to a method of producing curvature on the surface of a substrate through selective removal of material from the substrate and selective application of stress-producing materials to the substrate.
The use of lenses and mirrors to focus and direct visible light and other electromagnetic radiation is well established. Conventional lenses and mirrors, however, are not effective for focusing or directing electromagnetic radiation having short wavelengths, such as x-rays and short-wave ultraviolet rays. It is known that short wavelength radiation is strongly reflected from reflective surfaces if the angle of incidence to the surfaces is low, for example, less than five degrees for one nanometer (nm) or shorter wavelength x-rays. Employing such so-called grazing incidence techniques, however, is only effective if suitably uniform and smooth reflective surfaces can be found, and this has proved difficult to do. Use of conventional polished or mirrored surfaces, prepared using known grinding and polishing techniques, generally does not provide the desired control and accuracy in reflecting short wave radiation.
One approach to achieving better control and accuracy in reflecting x-rays, even at larger angles of incidence, is to use so-called Bragg reflection--reflection of radiation from planes of a crystal. The drawback of this has, until recently, been that only very short wavelength x-rays (less than one nanometer) could be reflected. Recently, however, multi-layered thin film structures have been employed for reflecting longer wavelength x-rays.
Such reflection occurs at the interfaces of adjacent films having different refractive indices. The layers can be selected so that reflected waves add constructively to produce a strong total reflection.
In order to focus electromagnetic radiation using reflection, it is necessary to provide an appropriate curved surface capable of reflecting the radiation. Currently used focusing devices are constructed either by lathing, grinding or otherwise abraiding a surface of a reflector element into a curved surface, or by casting a reflector element on a curved mandrel. Among the problems with these approaches are the difficulty of obtaining desired curvatures, the roughness of the resulting curved surface, and the time required to prepare the elements. Polishing, of course, would be performed to smooth out the surface, but remaining irregularities would preclude or discourage use of the surface for focusing shorter wave radiation such as x-rays.